On a heat curve, the temperature remains constant during a phase change. Between points B and C, the substance is undergoing a phase transition (e.g., melting or boiling), where energy is being added to the system but is used to change the phase rather than increase the temperature.
For example, if point B represents the melting of a solid to a liquid, the heat energy added is used to break the intermolecular forces holding the solid structure together, rather than increasing the kinetic energy of the molecules, which would raise the temperature. This is why you observe no temperature change from B to C.
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